Re: [RFC PATCH v4 1/7] mm/demotion: Add support for explicit memory tiers

[Ying Huang]

On Thu, 2022-06-02 at 14:07 +0800, Ying Huang wrote:
> On Fri, 2022-05-27 at 17:55 +0530, Aneesh Kumar K.V wrote:
> > From: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
> > 
> > In the current kernel, memory tiers are defined implicitly via a
> > demotion path relationship between NUMA nodes, which is created
> > during the kernel initialization and updated when a NUMA node is
> > hot-added or hot-removed.  The current implementation puts all
> > nodes with CPU into the top tier, and builds the tier hierarchy
> > tier-by-tier by establishing the per-node demotion targets based
> > on the distances between nodes.
> > 
> > This current memory tier kernel interface needs to be improved for
> > several important use cases,
> > 
> > The current tier initialization code always initializes
> > each memory-only NUMA node into a lower tier.  But a memory-only
> > NUMA node may have a high performance memory device (e.g. a DRAM
> > device attached via CXL.mem or a DRAM-backed memory-only node on
> > a virtual machine) and should be put into a higher tier.
> > 
> > The current tier hierarchy always puts CPU nodes into the top
> > tier. But on a system with HBM or GPU devices, the
> > memory-only NUMA nodes mapping these devices should be in the
> > top tier, and DRAM nodes with CPUs are better to be placed into the
> > next lower tier.
> > 
> > With current kernel higher tier node can only be demoted to selected nodes on the
> > next lower tier as defined by the demotion path, not any other
> > node from any lower tier.  This strict, hard-coded demotion order
> > does not work in all use cases (e.g. some use cases may want to
> > allow cross-socket demotion to another node in the same demotion
> > tier as a fallback when the preferred demotion node is out of
> > space), This demotion order is also inconsistent with the page
> > allocation fallback order when all the nodes in a higher tier are
> > out of space: The page allocation can fall back to any node from
> > any lower tier, whereas the demotion order doesn't allow that.
> > 
> > The current kernel also don't provide any interfaces for the
> > userspace to learn about the memory tier hierarchy in order to
> > optimize its memory allocations.
> > 
> > This patch series address the above by defining memory tiers explicitly.
> > 
> > This patch adds below sysfs interface which is read-only and
> > can be used to read nodes available in specific tier.
> > 
> > /sys/devices/system/memtier/memtierN/nodelist
> > 
> > Tier 0 is the highest tier, while tier MAX_MEMORY_TIERS - 1 is the
> > lowest tier. The absolute value of a tier id number has no specific
> > meaning. what matters is the relative order of the tier id numbers.
> > 
> > All the tiered memory code is guarded by CONFIG_TIERED_MEMORY.
> > Default number of memory tiers are MAX_MEMORY_TIERS(3). All the
> > nodes are by default assigned to DEFAULT_MEMORY_TIER(1).
> > 
> > Default memory tier can be read from,
> > /sys/devices/system/memtier/default_tier
> > 
> > Max memory tier can be read from,
> > /sys/devices/system/memtier/max_tiers
> > 
> > This patch implements the RFC spec sent by Wei Xu <weixugc@xxxxxxxxxx> at [1].
> > 
> > [1] https://lore.kernel.org/linux-mm/CAAPL-u-DGLcKRVDnChN9ZhxPkfxQvz9Sb93kVoX_4J2oiJSkUw@xxxxxxxxxxxxxx/
> > 
> > Signed-off-by: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> 
> IMHO, we should change the kernel internal implementation firstly, then
> implement the kerne/user space interface.  That is, make memory tier
> explicit inside kernel, then expose it to user space.

Why ignore this comment for v5?  If you don't agree, please respond me.

Best Regards,
Huang, Ying